The desired characteristics of a memory cell for computer main memory are high speed, low power, nonvolatility, and low cost. Low cost is accomplished by a simple fabrication process and a small surface area. Dynamic random access memory (DRAM) cells are fast and expend little power, but have to be refreshed many times each second and require complex structures to incorporate a capacitor in each cell. Flash type EEPROM cells are nonvolatile, have low sensing power, and can be constructed as a single device, but take microseconds to write and milliseconds to erase, which makes them too slow for many applications, especially for use in computer main memory. Conventional semiconductor memory cells such as DRAM, ROM, and EEPROM have current flow in the plane of the cell, i.e., "horizontal", and therefore occupy a total surface area that is the sum of the essential memory cell area plus the area for the electrical contact regions, and therefore do not achieve the theoretical minimum cell area.
Unlike DRAM, magnetic memory cells that store information as the orientation of magnetization of a ferromagnetic region can hold stored information for long periods of time, and are thus nonvolatile. Certain types of magnetic memory cells that use the magnetic state to alter the electrical resistance of the materials near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called magnetic RAM or MRAM.
In prior art MR memory cells based on the anisotropic magnetoresistive (AMR) effect, as described, for example, in Daughton, Thin Solid Films, Vol. 216, 1992, the cell resistance values are on the order of 10 to 100 Ohms using practical film materials and thicknesses. In the AMR effect, the electrical resistance of certain magnetic metals varies as the square of the cosine of the angle between the magnetization and the direction of the sense current. Because the sensing current direction through the AMR memory cell is horizontal, or in the plane of the films making up the cell, a long and narrow shape for the MR material is required to increase the resistance. Long and narrow shapes do not allow the cell to be drawn in as small an area as a DRAM cell. An alternative magnetoresistive effect, called giant magnetoresistance (GMR), which includes the "spin valve" type of GMR magnetic memory cell, also has current flow in the horizontal direction and similar resistance values. An MRAM based on spin valve GMR memory cells is described in IBM's U.S. Pat. No. 5,343,422.
For high capacity memories useful for computer main storage, a higher value of resistance is desired. A high resistance value for a magnetic memory cell is needed to reduce the sense power. Thus, the inherently low resistance of AMR and GMR memory cells, and the fact that their resistance must be increased by increasing their surface area appropriately, severely limits the use of these types of magnetic memory cells for high-density nonvolatile storage.
AMR and GMR memory cells also have power inefficiency due to the arrangement of the individual memory cell elements into an array. MRAM devices using AMR and GMR memory cells are organized as a series connection of many cells through which the sense current flows. When one memory cell in the series path is being sensed, the current flows through many other cells. This reduces the efficiency of the sensing process in two ways. First, sensing the value of the resistance of the selected cell is more difficult due to the series resistance of the many other cells in the sense path. Second, sensing power is higher due to the power dissipated in the many other cells in the sense path. Power efficiency is needed for high-capacity MRAM useful for computer main storage. Thus, the sensing inefficiencies of MRAM using AMR and GMR memory cells severely limits the use of these types of MR memory cells in high-capacity nonvolatile storage.
A magnetic tunnel junction (MTJ) is based on substantially different physical principles than AMR or GMR. In an MTJ, two ferromagnetic layers are separated by an insulating tunnel barrier and the magnetoresistance results from the spin-polarized tunneling of conduction electrons between the two ferromagnetic layers. The tunneling current depends on the relative orientation of the magnetic moments of the two ferromagnetic layers. An MTJ is described by Moodera et al. in "Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions", Physical Review Letters, Vol. 74, No. 16, 17 Apr. 1995, pp. 3273-3276. MTJ devices have several practical limitations that have prevented their commercialization, and no operable MRAM using MTJs has been proposed or built.